1. Field of the Invention
This invention relates to a projection lens in which magnification is used in the vicinity of 1/10 times and F-number is relatively bright and various aberrations are well corrected and which is inexpensive.
2. Description of the Prior Art
In recent years, a method has been adopted in which a solid image pick-up element is used as the scanning means in facsimile and this is disposed as a scanning light receiving element in the image plane to scan an original. In this case, where a projection lens is used to transmit the original to the solid image pick-up element, the following points are particularly required in the projection lens. Firstly, the F-number of the lens must be bright because it is desired to increase the exposure amount to the solid image pick-up element per unit of time in order that the original may be scanned at high speed by the use of such element and because it is desired to use a fluorescent lamp of the lowest possible illumination as the lamp for illuminating the original. Secondly, in order to make the facsimile apparatus compact, it is required that the interval between the original and the image plane be made as small as possible and the angle of view of the lens be made as wide as possible. Thirdly, a high resolving power is required because one solid image pick-up element is of the order of 15.mu.. Fourthly, it is necessary that the quantity of light be equal over the entire area of the solid image pick-up element and therefore, the off-axis opening efficiency of the lens must be 100%. Fifthly, it is necessary that the original surface be uniformly projected, namely, that the distortion be small. Sixthly, the lens must be made inexpensive to make the facsimile apparatus inexpensive.
As a projection lens, the so-called Tessar type one is known from U.S. Pat. No. 2,724,992, Japanese Laid-open Patent Application No. 48233/1979, etc. The present invention is a Tessar type projection lens which satisfies the above-noted requirements.